The present invention relates generally to λ<200 nm transmitting optical fluoride crystals and optical elements therefrom, and particularly to testing and making high quality optical fluoride crystals and lithography/laser elements with high fluoride purity and very low lead and oxygen contaminant levels.
The burden of the demands for improved performance of computers falls on the lithographic process used to fabricate the integrated circuit chips. Lithography involves irradiating a mask and focusing the pattern of this mask through an optical microlithography system onto a wafer coated with a photoresist. The pattern on the mask is thereby transferred onto the wafer. Decreasing the line-widths of the features on a given wafer brings about advances in performance. The enhanced resolution required to achieve finer line-widths is enabled by decreasing the wavelength of the illumination source. The energies used in lithographic patterning are moving deeper into the UV region. Optical components capable of reliable performance at these short optical microlithography wavelengths are required. Few materials are known that have a high transmittance at 193 nm and 157 nm and do not deteriorate under intense laser exposure. Fluoride crystals such as calcium fluoride and barium fluoride are potential materials with high transmittance at wavelengths <200 nm. Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light at and below 193 nm provide desirable benefits in terms of achieving smaller feature dimensions. Microlithography systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits and their manufacture. The commercial use and adoption of 193 nm and below vacuum ultraviolet wavelengths such as 157 nm has been hindered by the transmission nature of such deep ultraviolet wavelengths in the 157 nm region through optical materials. Such slow progression by the semiconductor industry of the use of VUV light below 175 nm such as the 157 nm region light has been also due to the lack of economically manufacturable blanks from optically transmissive materials and difficulties in manufacturing blanks which can be identified as high quality and qualified for their intended microlithography optical element and laser use. For the benefit of deep ultraviolet photolithography in the VUV 157 nm region such as the emission spectrum of the fluorine excimer laser to be utilized in the manufacturing of integrated circuits there is a need for below 200 nm wavelength transmitting optical fluoride crystals that have beneficial optical and highly qualified properties including good transmission below 200 nm and at 193 nm and 157 nm and that can be manufactured, tested, evaluated, measured and qualified for use economically. The present invention overcomes problems in the prior art and provides a means for economically providing high quality measured very low lead contaminant levels for below 200 nm wavelength transmitting optical fluoride crystals that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths. The invention provides for absorption band analysis testing of high quality calcium fluoride optical fluoride crystal lithography and excimer laser elements with very low lead contaminant levels.